Power-line-operated high frequency power supply

ABSTRACT

In an inverter-type power supply, a self-oscillating full-bridge inverter is powered by unfiltered full-wave-rectified 120 Volt/60 Hz power line voltage and controllably triggered into oscillation each half-cycle of the power line voltage by way of triggering two of the bridge&#39;s four transistors into conduction simultaneously. This is accomplished by way of discharging a charged-up capacitor by way of a Diac through a special trigger winding on each of two saturable current feedback transformers; each of which controls a pair of the bridge transistors.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 07/040,809filed Apr. 21, 1987; now U.S. Pat. No. 4,908,754, which is acontinuation of application Ser. No. 06/679,929, filed on Dec. 10, 1984,now U.S. Pat. No. 4,680,506.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to power-line-operated electronicinverter-type power supplies for microwave ovens.

2. Prior Art

Power-line-operated electronic inverter-type power supplies formicrowave ovens and other applications have been previously described,such as in Hester, U.S. Pat. No. 3,973,165, or in Kiuchi et al. U.S.Pat. No. 4,002,875. However, these previously described power supplieshave not integrally addressed, let alone resolved, several basic issuesassociated with the practical application of such power supplies. Theseissues relate to: (i) the power factor by which the power supply drawspower from the power line; (ii) the power factor at which the invertersupplies its output power; and (iii) the crest-factor of the currentsupplied to the magnetron, the crest-factor being the ratio of peak toaverage current.

BACKGROUND CONSIDERATIONS

In powering the magnetron in a microwave oven by way of apower-line-operated electronic inverter-type power supply, in order toachieve an acceptably good power factor in respect to the loadingrepresented by the inverter, as well as in respect to the loadingpresented to the inverter, it is desirable to extract the power from theinverter by way of a tuned circuit. Otherwise, the Volt-Ampere productthat must be supplied by the power line to the inverter, as well as theVolt-Ampere product that must be supplied by the inverter to themagnetron, get to be unacceptably large.

It is particularly desirable to power the magnetron by way of a high-Qresonant L-C circuit wherein the magnetron load is effectivelyparallel-connected across the tank capacitor of the L-C circuit, andwherein this L-C circuit is effectively series-connected across theinverter's output.

However, when such a high-Q series-excited resonant L-C circuit is notloaded, it acts in effect as a short circuit; which, if allowed to existfor even a very brief period, is apt to cause destructive overload ofthe inverter and/or the L-C circuit.

In an ordinary magnetron, a cathode must be heated to incandescencebefore electron emission starts and therefore before the magnetronbecomes conductive; and this heating process or pre-conditioning is aptto require from one to two seconds.

Thus, aside from the relatively modest amount of power needed toaccomplish the pre-conditioning, a magnetron is substantially anon-conducting load until its cathode has reached incandescence; whichimplies that, during this brief period of one to two seconds, the shortcircuit represented by the unloaded series-resonant L-C circuit is aptto cause destructive overload.

One way of preventing such destructive overload is that of connecting inparallel with the magnetron a voltage-limiting means (like a Varistor)characterized by: i) not conducting at the highest magnitude of voltagenormally present across the magnetron when it is conducting; and ii)conducting heavily at a voltage of somewhat higher magnitude than that.

However, due to the significant amount of energy that must be absorbedby this voltage limiting means, being on the order of 1000 to 2000 Joulefor a conditioning period of one to two seconds, the effective costassociated with such a method of preventing destructive overload ofinverter and/or L-C circuit is very high.

In respect to the current crest-factor, it is noted that prior-artinverter-type magnetron power supplies provide to the magnetronunidirectional current pulses at the relatively high frequency of theinverter, but with the magnitude of these pulses varying roughly inproportion with the instantaneous magnitude of the power line voltage.The crest-factor resulting from this double-modulation of the magnetroncurrent is particularly disadvantageous.

SUMMARY OF THE INVENTION Objects of the Invention

An object of the present invention is that of providing a basis fordesigning power-line-operated, high-power-factor, high-efficiency,cost-effective, inverter-type microwave oven power supplies.

Another object is that of providing an inverter-type power supply thatis operative to safely power a magnetron load that is parallel-connectedwith a series-excited high-Q resonant L-C circuit.

These as well as other important objects and advantages of the presentinvention will become apparent from the following description.

BRIEF DESCRIPTION

In its preferred embodiment, subject invention constitutes apower-line-operated electronic inverter-type power supply operable toprovide pulsed anode current to the magnetron in a microwave oven. Thispower supply comprises a full-wave rectifier providing its non-filteredpulsed DC voltage output to a full-bridge inverter operable toseries-excite a high-Q parallel-loaded resonant L-C circuit. Theparallel-loading is provided by the magnetron; which, however, is ofsuch nature as to have to be conditioned for a period of about twoseconds before becoming fully operative as a load. This pre-conditioninginvolves heating the magnetron cathode to the point of thermionicincandescence.

Power to heat the magnetron cathode is provided by way of a smalltransformer connected directly with the power line. However, to preventthe unloaded series-resonant L-C circuit from overloading the inverterduring the brief period it takes for the magnetron cathode to reachincandescence, the inverter is prevented from operating until after thecathode has reached incandescence.

To ascertain a good crest-factor in respect to the current provided tothe magnetron, filtering means have been provided by which highfrequency modulations of the magnetron current are substantiallyeliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic circuit diagram of the preferred embodimentof the invention.

FIG. 2 shows various voltage and current waveforms associated withdifferent aspects of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT Details of Construction

FIG. 1 shows an AC voltage source S, which in reality is an ordinary 120Volt/60 Hz electric utility power line.

Connected to S is a full-wave rectifier FWR that rectifies the ACvoltage from S to provide an unfiltered DC voltage between a positivepower bus B+ and a negative power bus B-.

A first pair of transistors Q1a and Q1b are connected in series betweenthe B+ bus and the B- bus in such a way that the collector of Q1a isconnected to the B+ bus, the emitter of Q1a is connected with thecollector of Q1b at a junction J1, and the emitter of Q1b is connectedwith the B- bus.

A second pair of transistors Q2a and Q2b are connected in series betweenthe B+ bus and the B- bus in such a way that the collector of Q2a isconnected to the B+ bus, the emitter of Q2a is connected with thecollector of Q2b at a junction J2, and the emitter of Q2b is connectedwith the B- bus.

Primary winding FTap of saturable feedback transformer FTa and primarywinding FTbp of saturable feedback transformer FTb are connected inseries between junction J1 and output terminal OT1. Another outputterminal OT2 is connected with junction J2.

A first secondary winding FTa1 of feedback transformer FTa is connectedbetween the base and the emitter of transistor Q1a; and a secondsecondary winding FTa2 of the same transformer FTa is connected betweenthe base and the emitter of transistor Q2a.

A first secondary winding FTb1 of feedback transformer FTb is connectedbetween the base and the emitter of transistor Q1b; and a secondsecondary winding FTb2 of the same transformer FTb is connected betweenthe base and the emitter of transistor Q2b.

Auxiliary winding FTax on transformer FTa and auxiliary winding FTbx ontransformer FTb are connected in series between the B+ bus and a controlinput terminal CI.

The full-bridge inverter assembly consisting of transistors Q1a, Q1b,Q2a and Q2b, as connected in circuit with feedback transformers FTa andFTb, is referred to as FBI.

Primary winding ATp of auxiliary transformer AT is connected with sourceS; and secondary winding ATs of transformer AT is connected withmagnetron cathode terminals MC1 and MC2 of magnetron M.

Primary winding MTp of main transformer MT is connected between outputterminals OT1 and OT2. An inductor L and a capacitor C areseries-connected directly across the secondary winding MTs oftransformer MT. Capacitor C is connected between capacitor terminals CT1and CT2, with CT1 being the point at which capacitor C is connected withinductor L.

Capacitor terminal CT1 is connected with the anode of a rectifier Ra andwith the cathode of another rectifier Rb. Two capacitors Ca and Cb areseries-connected between the cathode of rectifier Ra and the anode ofrectifier Rb. The two capacitors are connected together at capacitorterminal CT2.

The cathode of rectifier Ra is connected with the magnetron anode MA;and the anode of rectifier Rb is connected to magnetron cathode terminalMC1.

A capacitor C1 and a Diac D are connected in series between the B+ busand control input terminal CI. An auxiliary transistor Qa is connectedwith its collector to the junction between C1 and D, and with itsemitter to the B- bus. A resistor R1 is connected between the B+ bus andthe cathode of a Zener diode Z; and a parallel-combination of a resistorR2 and a capacitor C2 is connected between the cathode of Z and the B-bus. The anode of Z is connected with the base of Qa. A rectifier Rd isconnected with its cathode to the collector of Qa and with its anode tothe collector of Q1b.

Explanation of Waveforms

FIG. 2a shows the full-wave rectified 120 Volt/60 Hz power line voltageas present between the B- bus and the B+ bus.

FIG. 2b shows the trigger pulses provided to control input terminal IC.

FIG. 2c shows the high-frequency squarewave voltage provided betweeninverter output terminals OT1 and OT2.

FIG. 2d indicates the shape of the anode current provided to themagnetron.

FIG. 2e shows the corresponding current drawn by the inverter from thefull-wave rectifier.

FIGS. 2f-2h show the waveforms of FIGS. 2b-2d for the situation ofproviding delayed trigger pulses to the inverter.

Description of Operation

The operation of the arrangement of FIG. 1 may be explained as follows.

The full-bridge inverter is arranged with positive current-feedback byway of saturable current transformers FTa and FTb in such a way as toself-oscillate, provided however that a DC voltage of adequate magnitudeis present between the B+ bus and the B- bus and that a triggeringcurrent pulse has been provided through auxiliary windings FTax andFTbx.

Assuming the polarity of the triggering pulse to be negative (i.e.,equivalent to positive current flowing out of control input terminal CI)and adequate in magnitude, the effect of the triggering pulse is that ofmomentarily rendering transistors Q2a and Q1b conductive; which thenstarts current flowing from the B+ bus, through Q2a and the primarywinding of main transformer MT, in direction from J2 to J1, and thenthough Q1b to the B- bus. Hence, current starts flowing through primarywindings FTap and FTbp of the feedback transformers in such a directionas to perpetuate the conductive states of transistors Q2a and Q1b.

However, feedback transformers FTa and FTb are both saturable; and aftera brief period, both of these feedback transformers saturate, at whichpoint base current ceases to be provided to the two conductingtransistors Q2a and Q1b, thereby rapidly rendering them non-conductive.

Due to inductively stored energy (as stored in inductance associatedwith transformer MT and/or its load) current will continue to flow fromjunction J2 to junction J1 for some brief period after transistors Q2aand Q1b have ceased to conduct. This current will flow until theinductively stored energy has been discharged.

However, the path of this discharging inductive current will not bethrough transistors Q2a and Q1b. Rather, it will be through transistorsQ2b and Q1a: on the one side it will flow from the B- bus, through theFTb2 winding, through the base-collector junction of transistor Q2b, andto junction J2; while on the other side it will flow from junction J1,through winding FTa1, through the base-collector junction of transistorQ1a, and to the B+ bus.

Due to charge storage effects in the transistor junctions, and as aresult of the reverse current-flow through transistors Q2b and Q1a,these two transistors have both been rendered temporarily conductive intheir forward directions. Hence, both these transistors will beforwardly conductive for a brief period after the inductive dischargecurrent has stopped flowing therethrough. During this brief period,forward current starts flowing from the B+ bus, through transistor Q1a,from junction J1 through primary winding MTp to J2, through transistorQ2b, and then to the B- bus, thereby initiating a new self-sustaininginverter cycle.

Thus, as long as the magnitude of the DC voltage present between the B+bus and the B- bus is above a certain minimum level, the inverter willexhibit self-sustaining oscillations, thereby providing a squarewavevoltage between its output terminals OT1 and OT2.

Except for relatively small voltage drops across the transistors, theabsolute magnitude of this output squarewave voltage will be the same asthat of the DC voltage between the B+ bus and the B- bus. Thus, as themagnitude of this DC voltage varies, so does the magnitude of thesquarewave voltage.

However, if the magnitude of the DC voltage drops below a certainminimum level--which level might typically be about 15 Volt--theinverter is no longer capable of self-sustained operation, and theoscillation ceases.

Thus, since the DC voltage present between the B+ bus and the B-bus--being unfiltered full-wave-rectified 60 Hz AC voltage--consists ofa series of sinusoidally-shaped DC voltage pulses provided at a rate of120 Hz (see FIG. 1a), the inverter must cease its oscillation toward theend of each voltage pulse. Hence, for inverter output to be provided ona continuous basis, the inverter must be re-triggered into oscillationfor each and every one of the voltage pulses--as indicated by FIG.2b--with the resulting inverter output then being a series of periodicintermittent bursts of squarewave voltage, as illustrated in FIG. 2c.

By varying the timing of the trigger pulses relative to the phasing ofthe voltage pulses, the time period during which the inverter oscillatescan be varied in a manner identical with the way the conduction angle ofa Triac or SCR can be varied. By not providing trigger pulses at all,the inverter simply does not oscillate and no inverter output results.

FIGS. 2f, 2g and 2h depict a situation of reduced power output from theinverter, where the trigger pulses are provided approximately in themiddle of each of the DC voltage pulses.

Trigger pulses are provided to the inverter at the IC terminal by way ofthe trigger assembly consisting of transistor Qa, Diac D, Zener diode Z,resistors R1/R2 and capacitors C1/C2.

More particularly, as soon as power line voltage is applied to rectifierFWR, capacitor C2 starts to charge by way of current flowing throughresistor R1 from the B+ bus. After about one to two seconds, capacitorC2 has been charged up to a voltage high enough so that current startsflowing through Zener diode Z and into the base of transistor Qa; andfrom that point on, except for a small amount of leakage current flowingthrough resistor R2, all the current provided by R1 flows into the baseof Qa. With current flowing into its base, Qa now becomes conductive andstarts to charge capacitor C1 at a relatively rapid rate. As soon as thevoltage on C1 grows to sufficient magnitude, Diac D breaks down andprovides a brief pulse to the CI terminal, thereby starting the inverteroscillating.

The time constant associated with charging C1 to the point of Diacbreakdown is chosen to be about one milli-second from the beginning ofone of the sinusoidally-shaped DC voltage pulses provided from therectifier; with the result being to provide triggering pulses asindicated by FIG. 1b.

Rectifier Rd connected between the collector of Qa and the collector ofQ1b serves to keep C1 discharged and thereby to prevent additional (andredundant) pulses from being provided to the CI terminal as long as theinverter is oscillating.

Resistor R2 serves to discharge C2 after the whole power supply isdisconnected from the power line, thereby making certain that capacitorC2 is fully discharged whenever power line voltage is re-applied to thepower supply.

Thus, the function of the trigger assembly is that of providingrepetitive triggering of the inverter as required for its properfunctioning, in accordance with FIG. 2b, but not until about one or twoseconds after applying power line voltage to the whole power supply.

The voltage provided across the series-connected inductor L andcapacitor C from secondary winding MTs of main transformer MT issubstantially of the same shape as that provided across the inverteroutput terminals, but is larger in magnitude. The L and the C are chosensuch as to have relatively high Q-factors and to be substantiallyseries-resonant at the frequency of the squarewave inverter outputvoltage (which is approximately of 30 kHz fundamental frequency). Inresponse to this squarewave voltage, the current flowing through the L-Cseries-resonant circuit will be substantially sinusoidal of waveshapeand in phase with the fundamental frequency component of the squarewavevoltage. In turn, this means that--absent some form of loadingmeans--the AC voltage developed across capacitor C will be sinusoidal ofwaveshape and very large in magnitude.

In this connection, as an example based on a situation with idealcomponents, with an unloaded circuit Q-factor of 100 and with thevoltage provided across secondary winding MTs being about 1000 Volt, itis noted that the voltage developing across the capacitor would be about100,000 Volt. Of course, with real components, such a voltage magnitudewill not be reached; and the circuit will instead become non-linearand/or destructively overloaded.

Under normal conditions, the magnetron will represent a voltage-limitingload to the L-C circuit, thereby preventing the circuit from becomingnon-linear and/or overloaded. However, during the brief period beforethe magnetron's cathode reaches a point of substantial thermionicemission, the magnetron does not provide any loading.

This delay associated with enabling the magnetron's cathode to becomeincandescent represents the reason for providing a delay in providingtrigger pulses to the inverter.

However, due to the delay in starting the inverter, the magnetron willindeed provide an adequate load for the L-C circuit--the cathode havingbeen sufficiently heated by voltage from transformer AT by the time theinverter starts to operate.

The voltage present across C, and therefore across terminals CT1 andCT2, is provided to a voltage-doubling rectifier/filter means consistingof Ra, Rb, Ca and Cb. The output of this voltage-doublingrectifier/filter means consists of a series of periodic intermittent DCvoltage pulses occurring at the rate of 120 Hz. Due to the particularvoltage-current characteristics of the magnetron, the shape of thesevoltage pulses will be nearly trapezoidal.

On the other hand, as illustrated in FIG. 2d, the current provided tothe magnetron will be in the form of periodic intermittentunidirectional pulses of shape nearly identical to that of the DCvoltage pulses existing between the B+ bus and the B- bus.

In this connection, it is noted that the magnitude of the currentprovided to a constant-voltage load connected in parallel with thecapacitor in a series-resonant L-C circuit is approximately proportionalto the magnitude of the AC voltage provided at the input to theseries-resonant circuit. That is, the instantaneous magnitude of thecurrent provided to the load is roughly proportional to theinstantaneous magnitude of the AC voltage provided at the input to theseries-resonant circuit.

In other words, a high-Q series-resonant circuit voltage-fed from an ACsource and with its load connected in parallel with its tank capacitor,represents a highly attractive way to power a load such as a magnetron,which is nearly a constant-voltage load and ideally requires a constantcurrent power source. By way of such an arrangement, the currentprovided to the magnetron load will be proportional to the magnitude ofthe voltage applied; which is to say that, by way of the high-Q L-Ccircuit, the series-connected voltage source gets converted to a currentsource for a parallel-connected load.

Thus, in the circuit of FIG. 1, as long as the inverter oscillates, themagnitude of the resulting magnetron current (FIG. 2d) will be roughlyproportional to the magnitude of the DC voltage present between the B+bus and the B- bus (FIG. 2a); which makes the magnitude of the currentdrawn by the inverter (FIG. 2e) roughly proportional to the magnitude ofthis DC voltage; which, in turn, makes the instantaneous magnitude ofthe current drawn from the AC source S roughly proportional to theinstantaneous magnitude of the voltage provided therefrom.

Hence, the power factor of the Volt-Ampere product drawn from AC sourceS is very good.

To provide for proper voltage-doubling, as well as to minimizehigh-frequency (30 kHz and up) ripple (or magnitude modulations) of theunidirectional current pulses provided to the magnetron, it is importantthat capacitors CMa and CMb be of adequate energy-storing capability.However, with an inverter frequency of about 30 kHz, and with thetypical magnetron requiring an input power of approximately 1000 Watt,it is adequate if these capacitors store an amount of energy that issomewhat larger than the amount of energy drawn by the magnetron duringone complete cycle of the 30 kHz inverter voltage. Thus, capacitors CMaand CMb should each be capable of storing approximately 50 milli-Joule.

Otherwise, the following points should be noted.

(a) The power supplied to the magnetron depends on the timing or phasingof the trigger pulses provided to the inverter. In turn, the timing ofthese trigger pulses depends on the delay associated with the process ofcharging capacitor C1 to a voltage high enough to cause breakdown ofDiac D. The length of this delay can be adjusted over a wide range byadjusting the resistance of R1.

Hence, by making R1 an adjustable resistor, the amount of power providedto the magnetron may be adjusted over a wide range.

(b) If the magnetron current were provided from unfiltered rectified 30kHz AC voltage, then--in order to provide for a given amound of averagemagnetron power--it would be necessary that the peak magnitudes of theresulting 30 kHz current pulses be very much larger than the peakcurrent that results without when there is filtered rectification of the30 kHz voltage; which would therefore result in highly unattractivemagnetron current crest factor.

(c) Automatically controlled provision and/or phasing of the triggerpulses shown in FIGS. 2b and 2f can be accomplished in a number of wellknown ways, the details of which form no part of this invention.

(d) It is important that the length of time that it takes for thesaturable feedback transformers FTa and FTb to saturate be somewhatshorter than the period of the natural resonance frequency of theseries-combination of inductor L and capacitor C. Ideally, thetransformer saturation time should be such that when it is added to thetransistor's storage time, the total equals the period of the naturalresonance frequency.

(e) Tank inductor L of the L-C resonant circuit does not have to be anindividual component separate and apart from main transformer MT.Rather, in most situations, L can simply be an integral part of MT,represented by the leakage inductance of its secondary winding MTs.

(f) The overall operation of the magnetron power supply arrangement ofFIG. 1 may be summarized as follows.

(i) As soon as the power supply arrangement is connected with a propervoltage source, heating power starts being supplied to the magnetroncathode; which cathode will reach incandescence within a time period ofabout one second, thereby rendering the magnetron fully functioning.

(ii) Unfiltered full-wave-rectified power line voltage is applied to aninverter; and the high-frequency voltage output from this inverter isapplied, by way of a step-up voltage transformer, across aseries-resonant high-Q L-C circuit. The voltage developing, with thehelp of so-called Q-multiplication, across the tank capacitor of thisL-C circuit is used for application to the magnetron after rectificationby way of a voltage-doubling rectifier and filter assembly.

(iii) Due to the delay associated with providing trigger pulses, theinverter does not start operation until after the magnetron cathode hashad a chance to reach incandescence. Thus, when the inverter does startto operate, the magnetron is fully functional and ready to act as aneffective parallel-connected load on the series-excited resonant L-Ccircuit.

(iv) With the magnetron being powered from the inverter by way of aseries-excited parallel-loaded resonant L-C circuit, the power factorassociated with the inverter's Volt-Ampere output is very good.Moreover, as indicated by FIG. 2e, which shows the current drawn by theinverter from the full-wave power line rectifier (FWR), the power factorof the Volt-Ampere input to the overall power supply arrangement fromthe power line is likewise very good.

It is believed that the present invention and its several attendantadvantages and features will be understood from the preceedingdescription. However, without departing from the spirit of theinvention, changes may be made in its form and in the construction andinterrelationships of its component parts, the form herein presentedmerely representing the presently preferred embodiment.

I claim:
 1. An arrangement comprising:a power line providing an ACvoltage at a pair of power line terminals; the AC voltage having a firstfundamental period and a first fundamental frequency; and afrequency-converting power supply connected with the power lineterminals and operative to provide a high frequency output voltage at apair of power output terminals; the high frequency output voltage havinga second fundamental frequency; the second fundamental frequency beingsubstantially higher than the first fundamental frequency; thefrequency-converting power supply including:(i) a rectifier connectedwith the power line terminals and operative to provide a substantiallyunfiltered DC voltage at a set of DC terminals; the absoluteinstantaneous magnitude of the DC voltage being, at least during half ofsaid first fundamental period, substantially equal to that of the ACvoltage; and (ii) bridge inverter means connected in circuit between theDC terminals and the power output terminals; the bridge inverter meansbeing operative to provide the high frequency output voltage; theabsolute instantaneous magnitude of the high frequency output voltagebeing substantially equal to that of the DC voltage;whereby, at leastduring half of said first fundamental period, the absolute instantaneousmagnitude of the high frequency output voltage is substantially equal tothat of the AC voltage.
 2. The arrangement of claim 1 wherein the highfrequency output voltage is provided in the form of individual bursts ofsquarewave voltage separated by periods of zero magnitude; an individualburst occurring at least once during each of said first fundamentalperiods.
 3. The arrangement of claim 2 wherein: (i) each of said periodsof zero magnitude has a duration; and (ii) the bridge inverter meansincludes control means operative to permit control of this duration. 4.The arrangement of claim 1 wherein a load means is connected in circuitwith the power output terminals; the load means including a tuned L-Ccircuit.
 5. The arrangement of claim 1 wherein the absoluteinstantaneous magnitude of the high frequency output voltage issubstantially equal to zero except during periods when it issubstantially equal to the absolute instantaneous magnitude of the ACvoltage.
 6. The arrangement of claim 1 wherein the absoluteinstantaneous magnitude of the DC voltage is substantially equal to thatof the AC voltage.
 7. An arrangement comprising:a power line providingan AC voltage at a pair of power line terminals; the AC voltage having afirst fundamental period and a first fundamental frequency; and afrequency-converting power supply connected with the power lineterminals and operative to provide a high frequency output voltage at apair of power output terminals; the high frequency output voltage havinga second fundamental frequency; the second fundamental frequency beingsubstantially higher than the first fundamental frequency; thefrequency-converting power supply including:(i) a rectifier connectedwith the power line terminals and operative to provide a DC voltage at aset of DC terminals; the absolute instantaneous magnitude of the DCvoltage being substantially equal to that of the AC voltage; and (ii)inverter means connected in circuit between the DC terminals and thepower output terminals; the inverter means being operative to providethe high frequency output voltage by way of causing each one of thepower output terminals to be electrically and alternatingly connected,substantially without any intervening impedance and as long as the highfrequency output voltage is indeed being provided, with one of the DCterminals, thereby to cause the absolute instantaneous magnitude of thehigh frequency output voltage to be substantially equal to that of theDC voltage.
 8. An arrangement comprising:a power line providing an ACvoltage at a pair of power line terminals; the AC voltage having a firstfundamental period and a first fundamental frequency; and afrequency-converting power supply connected with the power lineterminals and operative to provide a high frequency output voltage at apair of power output terminals; the high frequency output voltage havinga second fundamental period and a second fundamental frequency; thesecond fundamental frequency being substantially higher than the firstfundamental frequency; the frequency-converting power supplyincluding:(i) a rectifier connected with the power line terminals andoperative to provide a substantially unfiltered DC voltage at a set ofDC terminals; the absolute instantaneous magnitude of the DC voltagebeing, at least during half of said first fundamental period,substantially equal to that of the AC voltage; and (ii) bridge invertermeans connected in circuit between the DC terminals and the power outputterminals; the bridge inverter means being operative to provide the highfrequency output voltage; the average absolute magnitude of the highfrequency output voltage, when averaged over the duration of said secondfundamental period, being substantially equal to that of the DC voltagewhen averaged over the same duration.
 9. A combination comprising:asource operable to provide a DC voltage having absolute instantaneousmagnitude substantially equal to that of an ordinary AC power linevoltage; the AC power line voltage having a first fundamental period anda first fundamental frequency; and a full bridge inverter circuitpowered from the DC voltage and operative to provide an output of highfrequency voltage; the high frequency voltage having a secondfundamental period and a second fundamental frequency; the secondfundamental frequency being substantially higher than the firstfundamental frequency; the high frequency voltage being provided in theform of individual bursts of high frequency AC voltage separated byperiods of zero magnitude voltage; the bursts occurring at a frequencyequal to a whole multiple of said first fundamental frequency.
 10. Thecombination of claim 9 wherein: (i) each of said periods has a duration;and (ii) the full bridge inverter circuit has control means operative topermit control of this duration.
 11. An arrangement comprising:a powerline providing an AC voltage at a pair of power line terminals; the ACvoltage having a first fundamental period and a first fundamentalfrequency; and a frequency-converting power supply connected with thepower line terminals and operative to provide a high frequency outputvoltage at a pair of power output terminals; the high frequency outputvoltage having a second fundamental frequency; the second fundamentalfrequency being substantially higher than the first fundamentalfrequency; the frequency-converting power supply including:(i) arectifier connected with the power line terminals and operative toprovide a DC voltage at a set of DC terminals; the absoluteinstantaneous magnitude of the DC voltage being substantially equal tothat of the AC voltage; and (ii) inverter means connected in circuitbetween the DC terminals and the power output terminals; the invertermeans being operative to provide the high frequency output voltage; theabsolute instantaneous magnitude of the high frequency output voltagebeing substantially equal to that of the DC voltage, except for a partof each of said first fundamental periods; during which part theabsolute instantaneous magnitude of the high frequency voltage issubstantially equal to zero.
 12. The arrangement of claim 11 wherein, aslong as the absolute instantaneous magnitude of the high frequencyvoltage is indeed equal to that of the AC voltage, one of the poweroutput terminals is electrically connected, substantially all of thetime and substantially without any intervening impedance, with one orthe other of the DC terminals.